1. Field of the Invention
The present invention relates to the field of hydrogen storage, controlled release of hydrogen that is stored and hydrogen storage method where molecular hydrogen is stored in micro-containers built of glass that is hollow glass microspheres.
The principal of the present invention is to recommend new glass systems and glass compositions and to identify new processes and develop novel techniques allowing for fast and effective hydrogen permeation through the wall of a glass or glass-ceramic microsphere which would permit to eliminate the existing barriers for hydrogen diffusion.
2. Background of the Art
The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the “Philosophical Magazine” Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1843. The fuel cell he made used similar materials to today's phosphoric-acid fuel cell. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulfonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the ‘Grubb-Niedrach fuel cell’. GE went on to develop this technology with NASA and McDonnell Aircraft, leading to its use during Project Gemini. This was the first commercial use of a fuel cell. It wasn't until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers which was demonstrated across the US at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt and Whitney licensed Bacon's U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks).
UTC's Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power has marketed this fuel cell as the PureCell 200, a 200 kW system.
Types of Fuel Cells
QualifiedWorkingFuel CellPowerTemperatureElectricalNameElectrolyte(W)(° C.)efficiencyStatusMetalAqueous alkaline?above −20?Commercial/Researchhydride fuelsolution (e.g.,(50% Ppeak @cellKOH)0° C.)Electro-Aqueous alkaline?under 40?Commercial/Researchgalvanic fuelsolution (e.g.,cellpotassium hydroxide)DirectPolymer membraneto 50 Wunder 40?Commercial/Researchformic acid(ionomer)fuel cell(DFAFC)Zinc-airAqueous alkaline?under 40?Mass productionbatterysolution (e.g.,potassium hydroxide)MicrobialPolymer membrane?under 40?Researchfuel cellor humic acidUpflow?under 40?Researchmicrobialfuel cell(UMFC)ReversiblePolymer membrane?under 50?Commercial/Researchfuel cell(ionomer)DirectAqueous alkaline? 70?Commercialborohydridesol. (e.g., NaOH)fuel cellAlkaline fuelAqueous alkaline10 kW tounder 80Cell:Commercial/Researchcellsol. (e.g., KOH)100 kW60-70%System:62%DirectPolymer membrane100 kW to 90-120Cell:Commercial/Researchmethanol(ionomer)1 MW20-30%fuel cellSystem:10-20%ReformedPolymer membrane5 W to(Reformer)250-Cell:Commercial/Researchmethanol(ionomer)100 kW30050-60%fuel cell(PBI)125-200System:25-40%Direct-Polymer membraneup to 140above 25 ??Researchethanol fuel(ionomer)mW/cm2 90-120cellFormic acidPolymer membrane? 90-120?Researchfuel cell(ionomer)ProtonPolymer membrane100 W to(Nafion)70-120Cell:exchange(ionomer) (e.g.,500 kW(PBI)125-22050-70%Commercial/ResearchmembraneNafion ® orSystem:fuel cellPolybenzimidazol30-50%fiber)RFC-RedoxLiquid electrolytes1 kW to??Researchwith redox shuttle10 MW& polymer membrane(Ionomer)PhosphoricMolten phosphoricup to 10150-200Cell:Commercial/Researchacid fuel cellacid (H3PO4)MW55%System:40%Co-Gen:90%MoltenMolten alkaline100 MW600-650Cell:Commercial/Researchcarbonatecarbonate (e.g.,55%fuel cellsodium bicarbonateSystem:NaHCO3)47%TubularO2−-conductingup to 100 850-1100Cell:Commercial/Researchsolid oxideceramic oxideMW60-65%fuel cell(e.g., zirconiumSystem:(TSOFC)dioxide, ZrO2)55-60%ProtonicH+-conducting?700?Researchceramic fuelceramic oxidecellDirectSeveral different?700-850Cell:Commercial/Researchcarbon fuel80%cellSystem:70%Planar SolidO2−-conductingup to 100 700-1000Cell:Commercial/Researchoxide fuelceramic oxideMW60-65%cell(e.g., zirconiumSystem:dioxide, ZrO2)55-60%The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)
For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these number represents the difference between the reaction's enthalpy and Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.
Fuel cells are not constrained by the maximum Carnot cycle efficiency as combustion engines are, because they do not operate with a thermal cycle. At times this is misrepresented by saying that fuel cells are exempt from the laws of thermodynamics, because most people think of thermodynamics in terms of combustion processes (enthalpy of formation). The laws of thermodynamics also hold for chemical processes (Gibb's free energy) like fuel cells, but the maximum theoretical efficiency is higher (83% efficient at 298° K) than the Otto cycle thermal efficiency (60% for compression ratio of 10 and specific heat ratio of 1.4). Of course, comparing limits imposed by thermodynamics is not a good predictor of practically achievable efficiencies. Also, if propulsion is the goal, electrical output of the fuel cell has to still be converted into mechanical power with the corresponding inefficiency. In reference to the exemption claim, the correct claim is that the “limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems”. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.
For a fuel cell operated on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and adding moisture to it. This reduces the efficiency significantly and brings it near to the efficiency of a compression ignition engine. Furthermore fuel cells have lower efficiencies at higher loads.
The tank-to-wheel efficiency of a fuel cell vehicle is about 45% at low loads and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure. The comparable NEDC value for a Diesel vehicle is 22%. It is also important to take losses due to production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen.
Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions. While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.
Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a micro combined heat and power (m-CHP) application. When the heat is captured, total efficiency can reach 80-90%. CHP units are being developed today for the European home market.
Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts. Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability. This equates to less than one minute of down time in a six year period.
A new application is micro combined heat and power, which is cogeneration for family homes, office buildings and factories. This type of system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of energy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90% (35-50% electric+remainder as thermal) Molten-carbonate fuel cells have also been installed in these applications, and solid-oxide fuel cell prototypes exist.
However, since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).
Conceptually, hollow glass microspheres are the most promising alternative to gas tank technology, providing the potential to store high density of molecular hydrogen, the freedom of shape for their container, low production costs as well as an inherently safe manner to store gaseous hydrogen on board of a vehicle. The most important problem concerning the practical application of glass microspheres as micro-containers for hydrogen is linked to the slow rates of hydrogen permeation trough the walls of a glass sphere. Currently, to extract hydrogen, the glass spheres must be heated to high temperatures since at room temperature the gas extraction process is sluggish. Finding suitable glass compositions and development of techniques for optimization of hydrogen permeability in glass will permit to eliminate current barriers for practical application of the glass micro-containers for storage of gaseous hydrogen on board of a vehicle.
Published U.S. Patent Application No. 20070259220 describes that there are a variety of hydrogen storing materials that may be heated to release hydrogen. Three exemplary hydrogen storing materials that are suitable for use with the present invention include among others metal hydrides, carbon nanostructures (e.g., nanotubes, fullerenes, etc.), and glass microspheres. Conventional forms of each of these hydrogen storage materials are known in the arts. Metal hydrides contain hydrogen that has been reacted with and chemically bound by metals. In simplified concept the metal hydride “soaks up” hydrogen into the metal alloy the way a sponge soaks up water, although the hydrogen is chemically bound and may be recovered by heating rather than by squeezing. Many metal hydrides contain hydrogen bonded thereto under high-pressure conditions that may be released by heating at lower pressure. Carbon nanotubes are tubes of carbon on the order of several nanometers in diameter that may adsorb and store hydrogen on their surfaces and within their tubular structure. Carbon nanotubes have a high hydrogen storage capacity per unit weight. Glass microspheres are hollow glass spheres that can be used to store hydrogen. The microspheres may be heated to increase the permeability of their walls to hydrogen and filled or charged with hydrogen in a high pressure hydrogen environment. Thereafter the microspheres may be cooled to lock the hydrogen inside. Recovery of hydrogen from the microspheres may be achieved by a subsequent heating to again increase the permeability of the sphere walls to hydrogen and allow it to be recovered from the interior void. In some embodiments of the invention, the hydrogen storing material may comprise sodium alanate (sodium aluminum hydride or NaAlH.sub.4), or a doped sodium alanate. Hydrogen may be recovered from various doped sodium alanate materials by heating to temperatures not greater than about 150° C. Doped sodium alanates for hydrogen storage are disclosed in related U.S. Patent Application Publication No. 20040009121 filed on Jun. 16, 2003 (Craig M. Jensen and Scott D. Redmond) entitled “Improved Methods For Hydrogen Storage Using Doped Alanate Compositions”, which is hereby entirely incorporated by reference. As discussed therein, one suitable dopant is {n5C5H5}2TiH2. Hydrogen may be recovered from this material by heating to a temperature not greater than approximately 100° C. Many alternate dopants are also disclosed including related dopants wherein the cyclopentadienyl ring structure is modified or substituted, and those wherein the titanium is replaced by another catalyst such as zinc or another transition element. In an exemplary embodiment the ratios of NaH:aluminum:titanium are approximately 0.7:1.0:0.1 or else the molar ratio of NaH is in the range of approximately 0.1 to 0.88, the molar ratio of dopant is in the range of approximately 0.04 to 0.3, and approximately 3 moles of sodium are removed from the material for each approximately 1 mole of dopant added to the material. In alternate embodiments of the invention, the hydrogen storing material may comprise a solid alkali metal alanate as disclosed in U.S. Pat. No. 6,106,801 by Bogdanovic. In one embodiments of the invention, the hydrogen storing material may include rare earth hydrides, or many other materials that are known. In any event, the hydrogen storage material may be inserted into the cassette, charged with hydrogen under pressure, and the cassette may be sealed for distribution and subsequent hydrogen recovery.
U.S. Pat. No. 5,840,440 (Ovshinsky) discloses a broad range of glass compositions that can be used to store hydrogen for fuel cells. Rather than encapsulation of the hydrogen, hydrogen storage material that is characterized by a density of hydrogen storage sites of greater than 1.2×1023/cc and more preferably greater than 1.5×1023/cc, corresponding to a specific capacity which is far in excess of conventional hydrogen storage materials. The material can be used as an electrochemical electrode, a gas phase storage alloy or a fuel cell.
A hydrogen accumulation in hollow 5-200 μm glass microspheres with 0.5-5 μm walls is described by S. P. Malyshenko and O. V. Nazarova. (see a paper titled: “Hydrogen Accumulation” published in Nuclear and hydrogen energetics and technology (in Russian), issue 8, pp 155-205, 1988). When under pressure at 200° C.-400° C., hydrogen diffuses intensely through the walls, fills in the microspheres and remains there under pressure after cooling. When heating the microspheres to the above temperatures at external hydrogen pressure of 500 atm, hydrogen weight content (wt. %) in the microspheres reaches 5.5%-6.0%. The hydrogen weight content can be even lower, if the external hydrogen pressure is lower. On heating to 200° C., about 55% of hydrogen contained in microspheres will be released. Accordingly, about 75% of hydrogen contained in microspheres will be released on heating to 250° C. At hydrogen storage in glass microspheres, its wall diffusion losses are about 0.5% per 24 hours. In the case when the microspheres are coated with metal films, diffusion losses of hydrogen at room temperatures can be 10 to 100 times lower. The main drawback of this method is in the fact that the microspheric accumulator cannot be charged at very high hydrogen pressures and high temperatures, because it makes the process hazardous due to the low tensile strength of glass, which is within 20 kg/mm2. This does not allow hydrogen weight content in the microspheres to be substantially higher than 6% (by weight).